The present invention relates generally to microwave transmission circuits and, more particularly, to a conductive post structure for providing isolation between microwave circuit components and transmission media.
The use of conductive posts extending from a top conductive plane to a bottom conductive plane, forming a parallel plate type of structure, provides attenuation for waveguide modes of propagation within the parallel plate structure. For example, multi-layered printed wiring boards and multi-layered Teflon stripline assemblies make use of through-holes, which are metallic plated, to attenuate waveguide modes of propagation within the board or stripline assembly. The plated through-holes or so called "plated vias" are, however, randomly arranged on the wiring board and are spaced a sufficient distance away from the desired transverse electromagnetic (TEM) mode transmission lines to prevent distortion of the transmission line characteristic impedance, Z.sub.0.
It is known that the top plane-to-bottom plane and/or a single-end grounded conductive post can be used in waveguide transmission lines to form bandpass and band rejection filters. The conductive posts are arranged periodically along an axis of wave propagation i.e., the Z axis. In these waveguide structures, the desired signal is customarily passed through the periodic post structure in a low order waveguide mode.
Microstrip is a type of TEM transmission line useful from DC to millimeter wave frequencies for implementing passive circuit functions and to serve as a signal interconnection medium for combining passive and active circuit functions into complex functional assemblies e.g. monolithic microwave integrated circuits (MMIC) subsystem assemblies. The microstrip geometry includes a thin ribbon "center" conductor located over and parallel to a single conducting ground plane. Generally, the thin ribbon conductor is attached to and supported above the conducting ground plane by a dielectric or microstrip substrate having a uniform thickness. The microstrip substrate lies atop or is formed integrally with the conducting ground plane. Because of the open nature of the microstrip center conductor, i.e. it is located atop the microstrip substrate and in contact with the surrounding air, a top conducting or ground plane forming shield, such as a metal housing cover, is typically provided to shield the microstrip from the outside world. Generally, the housing cover is spaced a sufficient distance away from the transmission lines so as not to require the entire assembly, i.e., the microstrip transmission medium and cover, to be electrically considered as having a complex unsymmetrical double dielectric stripline geometry.
An air cavity is formed between the microstrip center conductor and the top conducting plane of the housing cover. The air cavity allows the attachment of surface mount technology (SMT) components, for example, beam-leaded diodes, standard solder-on SMT parts, etc. to the top surface of the microstrip substrate. Further, to maximize microwave transmission circuit performance, holes are machined into the microstrip substrate which extend downward toward the bottom conducting ground plane for holding drop-in circuit components such as a MMIC chip carriers, flange and stud mounted transistors, switches, etc.
While microstrip is useful as a component mounting and interconnecting medium, it exhibits poor electrical isolation between components and/or the transmission medium due to the open nature of its top surface. Signal leakage or radiation occurs between adjacent transmission lines and components due to the inductive and capacitive coupling of the lines and components. At frequencies wherein the circuit cavity has natural resonances, strong waveguide mode-type coupling occurs across large distances within the cavity. These waveguide modes propagate between the microstrip ground plane and the top conducting plane or shield of the housing cover.
Prior attempts to solve this radiation problem have attempted to isolate the circuit components and transmission lines by milling narrow channels into the metal housings and placing the microstrip transmission lines and components therein. The channel dimensions are sized too narrowly to propagate the waveguide modes for the frequency bands desired so as to isolate the lines and components. However, this approach is very costly due to its time-consuming and labor-intensive nature of requiring precision custom-milled and fabricated housings and manual or die cut substrates for subsequent assembly. Alternately, where isolation and signal attenuation are not critical parameters, it is known to use a single microstrip substrate in a large cavity with waveguide mode absorbing material coated on the top ground plane and/or with properly located film or bulk absorbers arranged within the cavity. However, this creates a very large and bulky subsystem assembly.
There is therefore needed a structure which provides good isolation and signal attenuation within the air cavity formed between the microstrip substrate and housing cover as well as within the microwave substrate itself. The structure should be compact and be easy and inexpensive to fabricate and assemble.
The present invention meets these needs by providing a two-dimensional periodic conductive post array structure which can be used to suppress higher order wave propagation within a large microstrip housing assembly without attenuating the desired TEM mode of propagation. Substantially vertical conductive posts are spaced at appropriate wavelength periods in the X and Y directions on top of and through the microstrip substrate in a so called "waffle-wall" type configuration. The periodic post structure provides high microwave signal isolation between nearby active and passive circuit functions located on the microstrip substrate. The desired TEM mode microstrip transmission line is routed through the conductive post matrix to provide the signal, control and DC paths between circuit chips and other active and passive circuit functions.
It is an advantage of the present invention to arrange the conductive posts by pressing them into a periodic matrix of holes drilled through the microstrip substrate into the microcircuit housing base. A compression layer is inserted between the housing cover and a conductive foil layer lying atop the ends of the conductive posts to ground the posts. The housing cover, compression layer, and conductive foil assure that the conductive posts are properly grounded at each of their ends. This embodiment allows for full access to the microstrip line center conductor for placing parts, performing probe testing and tuning prior to hermetically sealing the housing cover to the housing base.
A further embodiment includes a housing cover having holes pre-drilled therein corresponding to the periodic matrix of holes formed through the microstrip substrate. The conductive posts are inserted into the holes of both the cover and substrate to form an interference fit. To aid the conductive post insertion, the posts can be, for example, formed of a metal sheet which is rolled into a cylindrical post configuration. This "roll post" can compress to form the interference fit. By inserting the roll posts into the holes of the housing cover and base, a grounding effect occurs on the circumference of the posts in contact with the hole side walls. This compensates for large tolerances which may occur when attempting to set the spacing of the cover from the substrate and the initial pin insertion length. Tapered drill holes can be provided in both the housing cover and microstrip substrate to allow easier post insertion.
Yet another embodiment reduces the impedance of the conductive posts by performing series resonance tuning of the assembly. A dielectric film is placed atop the ends of the conductive posts prior to the conductive foil, compression and housing cover layers. The dielectric film has a controlled thickness and dielectric constant which forms a capacitance in series with the inductance of the conductive posts. Proper selection of the dielectric thickness, dielectric constant, post diameter and post length, allows precise tuning of the structure to the desired resonant frequency.
A further embodiment includes the forming of a noncontact "air" coaxial capacitance gap between the circumference of the conductive posts at their upper ends and the sidewalls of the pre-drilled holes in the housing cover. The coaxial capacitance is used to series tune the post length remaining within the air cavity formed between the microstrip substrate and the housing cover. It is an advantage of this embodiment to provide a "clean" structure not requiring a dielectric film, conductive foil and associated compression layer. Further, it is also compatible with staggered or regional frequency tuning and direct DC grounding of selected posts.
Another embodiment forms a noncontact air coaxial capacitance gap between the circumference of the conductive posts at their lower ends and the sidewalls of the premachined holes in the microstrip substrate and housing base. This embodiment has the advantage of being able to form the housing cover with integrally coupled conductive posts. The posts can be formed as part of the housing cover using high-precision electron discharge milling (EDM) metal fabrication equipment. Further, the capacitive tuning holes in the microstrip substrate can be placed very accurately using standard computer-controlled machining equipment. The absence of the conductive posts extending upward from the microstrip substrate allows easier and simpler placement and tuning of the circuit components.
A still further embodiment forms the conductive posts integrally with the housing cover but only extending downward to near the surface of the substrate. Tuning is accomplished via the end dielectric gap capacitance which, for microstrip, is between the post end and the lower level metal microstrip ground plane. For co-planar waveguide (C.P.W.) and slot line, wherein the circuit ground plane is on the substrate's top side, then the end dielectric capacitance is formed in the air gap between the post end and the substrate's top metal surface ground plane. This embodiment has the advantage of providing a low-cost subsystem assembly.
In addition to rejecting propagation in the air cavity above the microstrip substrate, conductive posts formed of plated vias located throughout the substrate are arranged in a periodic matrix structure to prevent propagation within the substrate. The "in-substrate" array of posts can also be combined with the "above-substrate" matrix of conductive posts to fully isolate circuit functions. The in-substrate plated vias can also be tuned by etching an annular air gap between the top termination of the metal plated vias and the top surface metallization of the CPW, or slot line substrate to form a capacitance for providing series resonance. This has the advantage of providing increased high frequency isolation and/or to allow greater spacing between plated vias.
It should be noted that these substrates can be active semiconductor substrates as might be used for large "wafer scale" integrated microwave or micrometerwave subsystem assemblies.